Although horizontal gene transfer by plasmids and bacteriophages has long been known to disseminate key traits among bacterial populations, only recently has the contribution of mobile chromosomal elements been fully realized. Large chromosomal "islands" that confer pathogenic, symbiotic or metabolic traits have been described, some of which can be conjugated between bacteria. Other important integrated elements exist and the term CONSTIN has been proposed to describe those that are conjugal self-transmissible integrating elements. Studies of the distribution of these diverse elements, their ability to exchange genetic information and their potential to alter the phenotype of an organism by en bloc transfer of discrete gene functions, are still in their relative infancy. Some CONSTINs integrate site-specifically; others are less restricted in their choice of insertion site. The latter may profoundly influence chromosome structure and dynamics, and modulate gene function via mutational changes accompanying promiscuous integration. Recently, distinctive CONSTIN-like elements were identified in two pathogenic mycoplasmas, M. fermentans and M. capricolum. The two elements share features including the presence of conjugation-like genes, flanking direct repeats and a non-replicative extrachromosomal form. Importantly, most genes on these unusual elements lack recognizable homologs and a gene encoding a recognizable DNA integration enzyme is not present. It is anticipated therefore, that a "novel" enzyme performs this integration/excision function. Mycoplasmas typically occupy host niches for long periods during chronic infections in diverse vertebrates, and therefore may serve as important reservoirs for this "novel" element. This underscores the need to understand the dynamics of these elements both within chromosomes of mycoplasmas and in the context of the exchange of mobile gene pools within a population. Accordingly, the specific aims of this proposal are designed (i) to assess the distribution and genomic context of the M. capricolum element in host-related mycoplasmas; (ii) to test the hypothesis that the element is indeed capable of chromosomal integration, and (iii) to explore possible DNA-protein interactions within the termini of the element, as these are predicted to play a critical role in mobility. Completion of these goals will increase our understanding of the function of these novel elements and will provide a platform from which the long term objectives, of understanding their capacity for intra- or inter-species gene transfer, their contribution to the observed plasticity of mycoplasma genomes and their possible role in pathogenesis, can be explored.